Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

28. Carbohydrates

Monosaccharides - Forming Cyclic Hemiacetals

Organic Chemistry

28. Carbohydrates

Monosaccharides - Forming Cyclic Hemiacetals

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1:08 minutes

Problem 21b

Textbook Question

Draw the structures of the compound α-D-allopyranose.
Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

Verified step by step guidance

Understand the problem: The task is to draw the structure of α-D-allopyranose. Allose is the C3 epimer of glucose, meaning it differs from glucose only at the configuration of the hydroxyl group on carbon 3. Additionally, the term 'α' indicates the anomeric hydroxyl group is in the axial position (down) in the pyranose ring form.

Step 1: Start with the Fischer projection of D-glucose. Identify the configuration of the hydroxyl groups on carbons 1 through 5. For D-glucose, the hydroxyl groups are on the right side for C2, C4, and C5, and on the left side for C3.

Step 2: Modify the Fischer projection of D-glucose to create D-allose. Since allose is the C3 epimer of glucose, flip the hydroxyl group on carbon 3 from the left side to the right side. The configurations of the other carbons remain unchanged.

Step 3: Convert the Fischer projection of D-allose into the Haworth projection of α-D-allopyranose. To do this, cyclize the molecule by forming a bond between the hydroxyl group on C5 and the carbonyl group on C1. Ensure that the anomeric hydroxyl group (on C1) is in the axial position (down) to represent the α-anomer.

Step 4: Verify the stereochemistry of the pyranose ring. Ensure that the hydroxyl groups on C2, C3, and C4 are positioned correctly based on the Fischer projection of D-allose. For D-allose, the hydroxyl groups on C2 and C3 should be equatorial (up), while the hydroxyl group on C4 should be axial (down).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Epimer

An epimer is a type of stereoisomer that differs in configuration at only one specific carbon atom among a set of chiral centers. In the context of carbohydrates, epimers are important for understanding the structural variations between sugars, such as allose and glucose, which differ at the C3 position.

Pyranose Structure

Pyranose refers to a six-membered cyclic form of sugars that includes five carbon atoms and one oxygen atom. This structure is significant in carbohydrate chemistry as it represents the most stable form of many monosaccharides in solution, influencing their reactivity and interactions.

Anomeric Carbon

The anomeric carbon is the carbon atom in a sugar that was the carbonyl carbon in the open-chain form and becomes a new chiral center upon cyclization. In pyranoses, this carbon is crucial for determining the alpha or beta configuration of the sugar, which affects its biological properties and reactivity.

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Related Practice

Textbook Question

Draw the structures of the compound methyl α-D-galactopyranoside.

Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

Textbook Question

Draw the structures of the compound ethyl β-D-ribofuranoside.

Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

Textbook Question

Two structures for the sugar glucose are shown on page 914. Interconversion of the open-chain and cyclic hemiacetal forms is catalyzed by either acid or base.

(a) Propose a mechanism for the cyclization, assuming a trace of acid is present.

Textbook Question

Two structures for the sugar glucose are shown on page 914. Interconversion of the open-chain and cyclic hemiacetal forms is catalyzed by either acid or base.

(b) The cyclic hemiacetal is more stable than the open-chain form, so very little of the open-chain form is present at equilibrium. Will an aqueous solution of glucose reduce Tollens reagent and give a positive Tollens test? Explain.

Textbook Question

Without referring to the chapter, draw the chair conformations of

(a) β-D-mannopyranose (the C2 epimer of glucose).

Textbook Question

Without referring to the chapter, draw the chair conformations of

(b) α-D-allopyranose (the C3 epimer of glucose).

Textbook Question

Without referring to the chapter, draw the chair conformations of

(d) N-acetylglucosamine, glucose with the C2 oxygen atom replaced by an acetylated amino group.

Multiple Choice

In the formation of a cyclic hemiacetal from a monosaccharide, what type of reaction occurs?

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